Build a module with its dependencies in a multi-module Maven project

INTRODUCTION

The mechanism in Maven 4 that handles multi-module projects is referred to as the reactor. This part of the Maven core does the following:

• Collect all the available modules to build
• Sort the modules into the correct build order
• Select which modules to build
• Build the selected modules in order

Module collection starts from one aggregator project. That project defines the modules of which it consists using the <modules> element. This is a recursive process, so aggregators can have child modules that are aggregators themselves. A multi-module project is built from an aggregator pom that manages a group of submodules.
The child modules are regular Maven projects, and they can be built separately or through the aggregator pom.

Build only one module out of a multi-module project

This solution shows you how to build a specific Maven module in a multi-module Maven project. It may be useful to reduce the build time during development, and it’s particularly useful when the project you are working on has lots of modules and it takes a long time to finish the build.

Let’s first understand how we can build a module out of a multi-module project with an example and the same example we will use in further sections. Let’s say the project structure would look like this:

 ParentProject
    |- pom.xml
    |-----------> CommonLibraries
    |               |-> pom.xml
    |-----------> APIs
    |               |-> pom.xml

where the APIs module is dependent on the CommonLibraries module.

In the ParentProject pom.xml, add all the submodules inside the modules section:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  
  <!-- parent pom -->
  <groupId>com.test</groupId>
  <artifactId>ParentProject</artifactId>
  <packaging>pom</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>ParentProject</name>
  
  <!-- sub modules -->
  <modules>
    <module>CommonLibraries</module>
    <module>APIs</module>
  </modules>
</project>

and in the individual submodule CommonLibraries pom.xml, add the ParentProject in the parent section:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0  http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

	<!-- parent pom -->
    <parent>
        <artifactId>ParentProject</artifactId>
        <groupId>com.test</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <groupId>com.test</groupId>
    <artifactId>CommonLibraries</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>	
</project>

Similarly, for submodule APIs pom.xml. Also, add dependency of CommonLibraries in it.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0  http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

	<!-- parent pom -->
    <parent>
        <artifactId>ParentProject</artifactId>
        <groupId>com.test</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <groupId>com.test</groupId>
    <artifactId>APIs</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>com.test</groupId>
            <artifactId>CommonLibraries</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>
</project> 

To build only the CommonLibraries module, follow the below-mentioned steps.
First, cd into the ParentProject directory and then run the command:

mvn -pl CommonLibraries clean install

Or, use the below command

mvn -f CommonLibraries/pom.xml clean install

 -f,--file                     Force the use of an alternate POM file.

 -pl,--projects          Comma-delimited list of specified reactor projects to build instead
                                   of all projects. A project can be specified by [groupId]:artifactId
                                   or by its relative path.

Building a module with its dependencies in a multi-module Maven project

As mentioned earlier, the APIs module is dependent on the CommonLibraries module.

The trick here is to use Maven advanced reactor options, particularly the following options:

mvn clean install -pl APIs -am

-am, –also-make                Builds the specified modules, and any of their dependencies in the reactor. 

You can customize the CI/CD tools like Jenkins to build only specific modules and their dependent modules using this command in the pipeline.

-am option only work with -pl option. It won’t work with other options like -f.

This solution makes it possible to decrease the build time, thus leading to decreased development time too, which may be a good way to make the development faster and cheaper.